Although Barker began in the early 1970s as a Marxist satirical playwright, by 2005 his approach had shifted in focus to such an extent that he felt able to define his theatre as having ‘many of the characteristics of a religion.’ This study investigates the relationship between Barker’s theatre and religious and spiritual ideas, focusing on two key influences: the medieval Christian mystical theologian Meister Eckhart and religious and mythic elements of ancient Greek tragedy. Barker’s dramatic engagement with Abrahamic monotheism reveals his interest in early biblical portrayals of God and his appropriation of dominant Christian tropes, notably apocalypse and rebirth. The specific influence of Eckhart’s apophatic theology, his Neoplatonic conception of the One and his doctrine of ‘detachment’ are shown to inform aspects of Barker’s work, including his theoretical text Death, The One and the Art of Theatre. Greek tragedy is examined as a religious and ritual event, establishing parallels with Barker’s view of tragedy as a sacred art that challenges rational and moral ideals by generating ecstatic emotions through an imagined proximity to death. Greek narratives that centre on an encounter with the dead, nekyia and katabasis, are explored in connection with Barker’s drama, along with ritual initiation in Greek mystery cult. Finally there is an investigation into the immoral, ecstatic, erotic, and thanatic aspects of the female protagonist in Greek tragedy and how these aspects of the tragic female continue and are appropriated in Barker’s contemporary tragedy. Eckhartian mystical theology and elements of classical tragic spirituality help to give Barker’s theatre a unique and mysterious dimension. The recurring antagonistic female archetype of ‘the one’ in Barker’s drama expresses core aspects of this spirituality: sexual ecstasy, proximity to death, and detachment from morality and ideology.